Casehardening



Patented Dec. 5, 1944 .CASEHARDENING Donald A. Holt, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application October 18, 1943, Serial No. 506,753

13 Claims.

This invention relates to case hardening steel and more particularly to case hardening steel by treatment in a fused salt bath containing alkali metal cyanide.

A common method of case hardening steel consists in treating steel in a fused bath of alkali metal cyanide, with or without diluent salts such as alkali metal carbonates, halides and the like.

-Such a bath gives a carburized case having a relatively high nitride content. For many purposes, it is desirable to decrease the amount of nitriding and increase the amount of carburizing in such operations. It is also desirable to accelerate the carburizing activity of such baths, to obtain deeper cases in shorter time of treatment.

One object of the present invention is to provide an improved'method of case hardening by means of a fused cyanide bath. A further object is to modify a cyanide case hardening operation so as to decrease the nitriding efiect and increase and accelerate the carburizing effect. A further object is to provide case hardening compositions and case hardening baths suitable for obtaining the above objects. Still other objects will be apparent from the following description of my invention.

The above objects may be attained in accordance with the present invention by the addition of a small amount of titanium cyanonitride, ti-

tanium carbide or a mixture thereof to a molten salt bath containing alkali metal cyanide. Titanium cyanonitride (also known as titanium carbonitride) is a known commercial product comprising a compound of titanium with varying amounts of carbon and nitrogen, as described in the textbook Titanium by W. M. Thornton,

Chemical Catalog Co., 1927, page .53.

Typical analysis of a commercial titanium cyano'nitride follows:

Per cent' Ti 80.0

N 1 6.8 O 6.6 Fe and other impurities 4.2

not less than 10% by weight.

tain a cyanide concentration of 20 to 30% by weight and to dilute the bath with alkali metal in a wide range. Thus, for the purpose of defining my invention, titanium cyanonitride may be considered as a compound of titanium with variable amounts of carbon and nitrogen and it may be pure or more or less contaminated with oxide or other impurities.

In practicing my invention, I prepare a molten alkali metal cyanide bath of the conventional type containing 10 to 50% by weight of cyanide and add to this a small amount of titanium cyanonitride or titanium carbide as activator. I prefer the activator in finely-divided form, for example, 100 mesh size or smaller, in an amount from 1 to 10% by weight of the bath. Generally I have obtained the best results by adding 2 to 5% by weight of the activator in finely-divided form. However, it is not essential to add finely-divided material, as I may obtain equivalent results by adding granular material of relatively large mesh size or even large pieces or briquets of cyanonitride but in such cases larger amounts usually are required. I have found that when a large amount of the activator is added, a relatively small amount, for example, in the neighborhood of around 5% by weight, dissolves in the cyanidebath, while the excess slowly dissolves and maintains continuous bath activity.

In order to obtain good results, I have found it necessary to maintain in the case hardening bath an alkali metal cyanide concentration of I prefer to maincarbonates or halides or both. However, if desired, pure alkali metal cyanide may be used to practice my invention. I prefer to utilize case hardening baths which are substantially free from alkaline earth metal compounds, including magnesium compounds or which contain not more than about 5% by weight of alkaline earth metal compounds.

The temperature of the bath preferably is maintained as in conventional cyanide case hardening operations, for' example, at 700 to 900 C. For example, I have obtained excellent results by operating the bath to case harden steel case in a shorter time than a plain cyanide bath.

In a preferred mode of practicing my invention, I melt a mixture of sodium cyanide and sodium carbonate containing about 30% of the cyanide and bring it to the temperature desired for case hardening, for example, at 820 to 850 C., and then add thereto 1 to 2% by weight of finely-divided titanium cyanonitride which has been screened to finer than 100 mesh. The bath is analyzed for cyanide from time to time and cyanide is added as required to maintain the cyanide concentration at 20 to 25% by weight. Also, during the operation the activity of the bath is maintained by hourly additions of from 0.01 to 0.10% by weight of the titanium cyanonitride.

For replenishing the bath, I prefer to prepare a composition containing from 60 to 98% of sodium cyanide and 2 to 40% by weight of titanium cyanonitride or titanium carbide, with or without diluent salts such as alkali metal halides or carbonates and to add this composi tion at hourly intervals or as required to maintain the desired degree of activity caused by the addition of the cyanonitride. As such replenishing mixture, I generally prefer to use a mixture containing approximately 10 parts by weight of sodium cyanide to one part by weight of titanium cyanonitride.

Example 1 A mixture of sodium cyanide and sodium carbonate containing 30% by weight of the cyanide was melted and bars of S. A. E.-1020 steel were carburized by treating the molten bath for 1 hour at 840 C. This bath had a low cyanate content and during the operation described below, an effective scum was maintainedon the bath surface to prevent increase in the cyanate' content. The bath was then activated by the addition of 2% by weight of titanium cyanonitride which had been ground to 100 mesh size or finer and other bars of the same steel then were treated in the bath for 1 hour.

The carbon and nitrogen gradients of 'the treated bars then were determined by cutting successive layers 0.004 inch thick from the bars and analyzing each layer for carbon and nitrogen. While the bars were being treated, samples of the bath were takenand analyzed for cyanide and cyanate. The following results were ob- A molten bath was prepared as.in Example 1, containing about 2% by weight of titanium cyanonitride. This bath was operated over a period of two days at a temperature of about 840 C. to carburize S. A. E.1020 steel bars, the bars being treated for one hour in the bath. Carbon and nitrogen gradients and the cyanide pontent of the bath were determined as in Example 1. The following results were obtained:

Steel bar samples A B 0 Cut Pcr- Per- Per- Per- Per Percent cent cent cent cent cent C N: C C N 1 0.71 0. 55 0. 75 0.68 0.70 0. 4a 2 h 0.03 0. 09 0.61 0. 09 0. 74 0.10 3 0. 43 0.02 0. 39 0. 04 0. 47 0.03 4. 0. 28 0. 01 0. 30 0. 02 G. 33 0. 0i 5.... 0.24 0.24 0.25 6. 0.23 0.23 0.22

Percent NaCN in bath 24. 86 24. 37 25. 59 Percent NaCNO in bath. l. 26 0. 92 1. 14

Example 3 A molten cyanide bath was prepared as in Example 1 and activated by the addition of 2% by weight of titanium carbide. This bath was operated over a period of 5 days, during which time sodium cyanide was added as required to maintain the sodium cyanide content at approximately 23% by weight, and bath activity was maintained 7 by hourly addition (during daily 8 hours of operation) of 0.02% by weight of titanium carbide. Carbon and nitrogen gradients were determined as in the preceding example. Following are typical results obtained in the treatment of S. A. E.- 1020 steel bars; the data being arranged chronologically, from left to right:

Steel bar samples A B C D Out Per Per Per Per Per Per Per Per cent cent cent cent cent cent cent cent N: C N: 0 N1 0 N] An advantage of my invention is that it produces a deeper carbon case than a plain cyanide bath and in shorter time. At the same time, the solidified bath adhering to treated work is entirely water-soluble and easily washed off, the washing characteristics being equally as good as I the ordinary plain cyanide bath.

-A further advantage, not possessed by plain unactivated cyanide baths, is that the bath may be operated with a protective covering of graphite or carbonaceous scum floating on the bath surface, which protects the bath from the air. The use of such protective floating layers is very desirable, having a number of advantages and represents a preferred mode of practicing my invention. The protective layer decreases the formation of disagreeable fumes of finely-divided sodium carbonate which often arise from cyanide baths. More important, it, decreases heat radiation from the bath, thus resulting in a saving of fuel rEuired to heat the bath and often prevents overheating, thus materially increaslng'the effec- 75 tive life of the bath container and furnace. The

protective layer, by exclusion oi air, also greatly decreases the rate of cyanide decomposition, thus making the operation more economical and facilitating control of cyanide concentration. In opcomprises treating steel articles in a molten bath containing 10 to 50% by weight of alkali metal cyanide and a small amount of a titanium compound selected from the group consisting of titanium cyanonitride and titanium carbide. the remainder of the bath being composed chiefly of at least one salt selected from the group consisting of alkali metal carbonates and halides.

2. The process for case carburizing steel which comprises treating steel articles in a molten bath containing 10 to 50% by weight of alkali metal cyanide the remainder composed chiefly of at least one salt selected from the group consisting of alkali metal carbonates and halides and a I small amount of a titanium compound selected from the group consisting of titanium cyanonitride and titanium carbide, said bath containing not more than about by weight of alkaline earth metal compounds and being maintained at a temperature of about 700 to 900 C.

3. The process for case carburizing steel which comprises treating steel articles in a molten bath containing about 20 to 30% by weight of sodium cyanide the remainder composed chiefly of at least one salt selected from the group consisting of alkali metal carbonates and'halides and about 1 to by weight of titanium cyanonitride, said bath containing not more than about 5% by weight of alkaline earth metal compounds and being maintained at a temperature of about 700 to 900 C.

4. The process for case carburizing steel which comprises treating steel articles in a molten bath containing about to 30% by weight of sodium cyanide the remainder composed chiefly of sodium carbonate and about 1 to 10% by. weight of titanium carbide, said bath containing not more than about 5% by weight of alkaline earth metal compounds and being maintained at a temperature of about 700 to 900 C.

5, The method for activating a molten alkali metal cyanid case carburizing bath composed chiefly of at least one salt selected from the group consisting of alkali metal oyanides, carbonates, and halides which comprises adding thereto a small amount of a titanium compound selected from the group consisting of titanium cyanoni tride and titanium carbide.

6. The method for activating a molten alkali metal cyanide case carburizing bath composed chiefly of at leastbne salt selected from the group consisting of alkali metal cyanides, carbonates, and halides which comprises adding thereto titanium cyanonitr'ide at a rate sufilcient to maintain a concentration of at least 1 to 5% by weight of said cyanonitride in said bath.

7. The method for activating a molten alkali metal cyanide case carburizing bath composed chiefly of at least one salt selected from the group consisting of alkali metal cyanides, carbonates, and halides which comprises adding thereto titanium carbide at a rate sufiicient to maintain a concentration of at least 1 to 5% by weight of said carbide in said bath. r

8. A case, carburizing composition comprising 10 to 50% by weight of alkali metal cyanide and a small amount of a titanium compound selected from the group consisting of titanium cyanonitride and titanium carbide, the remainder of said composition being composed chiefly of at least onesalt selected from the group consisting o alkali metal carbonates and halides.

9. A case carburizing composition comprising about 20 to 30% by weight of. alkalimetal cyanide and l to 5% by weight of titanium cyanonitride, the remainder of said composition being composed chiefly of at least one salt selected from the group consisting of alkali metal carbonates and halides.

10. A case carburizing composition comprising about 20 to 30% by weight of alkali metal cyanide and 1 to 5% by weight of titanium carbide, the remainder of said composition being composed chiefly of at least one salt selected from the group consisting of alkali metal carbonates and, halides.

11. A composition for replenishing and activating a cyanide case carburizing bath comprising to 98% .by weight of alkali metal cyanide and 2 to 40% by weight of a titanium compound selected from the group consisting of titanium 'cyanonitride and titanium carbide.

12. A composition for replenishing and activating a cyanide case carburizing bath consisting essentially of a mixture of approximately 10 parts by weight of sodium cyanide and one part by weight of titanium cyanonitride. v

13. A composition for replenishing and activating a cyanide case carburizing bath consisting essentially of a. mixture of approximately 10 parts by weight of sodium cyanide and one part by weight of titanium carbide.

. DONALD A. HOLT. 

